1. Field of the Invention
The present invention relates to a comparator of a differential amplifier type with a CMOS structure, and in particular, to a comparator that performs a protection function for a gate oxide film of an input MOS-FET.
2. Description of the Related Art
Widely used comparators compare a reference voltage Vref and an input voltage Vin and deliver an output voltage Vout corresponding to the relative magnitude. FIG. 8 shows a schematic construction of a conventionally common comparator of a differential amplifier type having a CMOS structure. This comparator is basically composed of a main body comparator 10 of a differential amplifier type and an inverter circuit 20 that inverts the output voltage of the main body comparator 10 and outputs the voltage.
The main body comparator 10 comprises a differential pair of input MOSFETs 11 and 12 of a p channel type that receive a reference voltage Vref and an input voltage Vin at respective gate terminals thereof. The source of each of the MOSFETs 11 and 12 is connected to a constant current source, and the drains are connected to load MOSFETs 13 and 14 of an n channel type, which are active loads. These MOSFETs construct a differential amplifier of a CMOS structure.
The load MOSFETs 13 and 14 have the source electrodes thereof grounded and the gate electrodes connected to each other. The drain electrodes thereof are connected to the drain electrodes of the pair of input MOSFETs 11 and 12. Of the load MOSFETs 13 and 14, the load MOSFET 13 in the side of receiving the reference voltage Vref has the gate and drain terminals connected to each other to be used as a diode. The constant current source is composed of a MOSFET 15 of a p channel type to supply constant current to the source terminals of the pair of input MOSFETs 11 and 12. The MOSFET 15 has the source electrode thereof connected to a power supply V and a gate terminal thereof receiving a specified bias voltage Vbias.
The inverter circuit 20 is composed of a MOSFET 21 of a p channel type and a MOSFET 22 of an n channel type that are in cascade connection and disposed between the power supply V and the ground (GND). The inverter circuit 20 having a CMOS structure composed of the MOSFETs 21 and 22 receives the drain voltage of the input MOSFET 12, which is the output of the main body comparator 10. The MOSFETs 21 and 22 turn ON complementarily and invert the output, which is a drain voltage of the MOSFET, and deliver the voltage Vout externally.
In the comparator having the construction described above, when the input voltage Vin is lower than the reference voltage Vref, Vin<Vref, the input MOSFET 11 receiving the reference voltage Vref is in an OFF state and the input MOSFET 12 receiving the input voltage Vin is in an ON state. As a result, the drain voltage of the input MOSFET 11 becomes approximately ground potential, nearly zero volts, turning OFF the load MOSFETs 13 and 14. The drain voltage of the input MOSFET 12, which is the output of the main body comparator 10, turns to an H level. As a consequence, the MOSFET 21 of the inverter circuit 20 turns OFF and the MOSFET 22 turns ON, turning the output voltage Vout of the inverter circuit 20 to a level which is the ground potential, zero volts.
When the input voltage Vin is higher than the reference voltage Vref, Vin>Vref, the input MOSFET 11 receiving the reference voltage Vref is in an ON state and the input MOSFET 12 receiving the input voltage Vin is in an OFF state. As a result, the load MOSFETs 13 and 14 assume an ON state and the drain voltage of the input MOSFET 12, which is the output of the main body comparator 10, becomes the ground potential, nearly zero volts, through the load MOSFET 14 in an ON state. As a consequence, the MOSFET 21 of the inverter circuit 20 turns ON and the MOSFET 22 turns OFF, turning the output voltage Vout of the inverter circuit 20 to an H level, which approximately equals the power supply voltage.
Patent Document 1 (identified further on), for example, discloses in detail a comparator that has a construction similar to the one described above and performs inversion operation corresponding to an input voltage Vin.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. S55-104766
A comparator having a construction as described above has an advantage in that the input impedance thereof is high owing to the construction of the input stage consisting of the input MOS-FETs 11 and 12, on the one hand. But the comparator has a disadvantage in that the permitted range of the input voltage Vin to the comparator is restricted by the thickness of the gate oxide film in the input MOSFETs 11 and 12 on the other hand. If a MOSFET with a thin gate oxide film is subjected to a high gate-source voltage Vgs, the gate oxide film breaks down due to the strong electric field.
A generally-used gate oxide film of SiO2 has a breakdown electric field strength of about 2.0×106 V/cm. Thus, a comparator capable of direct comparison of a high input voltage Vin can be constructed using a MOSFET having a sufficiently thick gate oxide film. A MOSFET with a thick gate oxide film, however, has difficulty in constructing a fine structure of the comparator in an integrated circuit structure. Further problems should arise of increase in the threshold voltage of the MOSFET and decrease in the switching speed.
FIG. 9 shows a simulation result of variation of gate-source voltages Vgs of the input MOSFETs 11 and 12 when the input voltage Vin is changed from zero to VDD in a comparator having a construction described above, wherein the power supply voltage VDD=42 V and the reference voltage Vref=VDD/2=21 V. In this example, the voltage between the gate and source of the input MOSFET 12 is the direct voltage difference between the gate of the input MOSFET 11 and the gate voltage of the input MOSFET 12, and rises to ±VDD/2=21 V at the maximum.
If the reference voltage Vref is set at a low voltage of the ground potential, zero volts, or at a high voltage near the power supply voltage VDD, the structure between the gate and source of the input MOSFET 12 can be subjected to a high voltage near the power supply voltage VDD at 42 V. Thus, a thin gate oxide film of the input MOSFETs 11 and 12 may break down.
In order to avoid this problem, a conventional comparator is generally provided with series-connected shunt resistors R1 and R2 as indicated in FIG. 8 to receive the input voltage Vin′ after dividing it with the resistors. The use of the shunt resistors R1 and R2, however, raises new problems of not only expansion of the circuit area for the comparator in an integrated circuit structure, but also degradation of accuracy due to the scattering of the shunt resistor ratio and malfunctioning due to degradation of the SN ratio.